This invention pertains to catalytic hydroconversion of petroleum residua feedstocks to produce lower boiling hydrocarbon liquid products. It pertains particularly to a catalytic hydroconversion process in which the reaction zone effluent is quenched to a temperature below about 775.degree. F. using a specific hydrocarbon material fraction, so as to avoid precipitation of asphaltene compounds in downstream processing equipment and provide sustained high conversion operations.
When heavy oil feedstocks such as crude petroleum oil, atmospheric residuum, vacuum residuum, or tar sands bitumen are hydrogenated in an ebullated bed catalytic reactor, the operating temperature is usually maintained above about 750.degree. F., with a typical reaction temperature being in the range of 800.degree. F. to 850.degree. F. When the reactor hot effluent stream is withdrawn from the reactor, the resulting liquid stream is normally quenched by direct injection of oil to cool the effluent stream to approximately 750.degree. F., so as to stop the thermally instigated reactions which subsequently cause product degradation and/or coke formation. However, it has been found that such quenching of the hot hydrocarbon effluent material can often cause undesirable precipitation of asphaltene compounds in downstream processing equipment, which causes serious operational difficulties in the process.
The catalytic hydrogenation of petroleum residua in an ebullated-bed reactor is well known. For example, in U.S. Pat. No. Re. 25,770 to Johanson, a process is disclosed whereby an ebullated bed catalytic reactor is used to accomplish hydroconversion of hydrocarbon feed material boiling above 975.degree. F. in an expanded catalyst bed, to produce lower boiling distillates, the catalyst particles being maintained in random motion by upward flow of the reactants. The recycle of hydrocarbon reactants boiling above about 680.degree. F. to the reaction zone is disclosed in U.S. Pat. No. 3,412,010 to Alpert, et al, wherein the recycle of such heavy fractions permits operation at higher levels of conversion of the 975.degree. F..sup.+ material. Also, moderate conversion of petroleum residua feedstocks to remove asphaltenes prior to desulfurization is disclosed in U.S. Pat. No. 3,948,756 to Wolk et al.
It has been known that operations on petroleum residua feedstocks at high hydroconversion levels, i.e., above about 75 V %, are not sustainable when the depressurized vaporous and liquid effluents from the catalytic reactor are permitted to mix under conditions of cooling to below about 750.degree. F. as is disclosed in U.S. Pat. No. 3,338,820 to Wolk et al. However, it has been observed that for conversions above about 85% this arrangement does not result in sustained operations. These high conversion reaction conditions cause precipitation of asphaltenes in a meso-phase which fouls and can even plug the downstream equipment, and when recycled to the reactor such asphaltenes cause the catalyst bed to agglomerate and defluidize. A long-sought solution to this asphaltene precipitation problem is advantageously provided by the present invention.